Intact Parathyroid Hormone Levels during Pregnancy, in Healthy Term Neonates and in Hypocalcemic Preterm Infants

Abstract

We measured parathyroid hormone levels in pregnant and nonpregnant women and at 1, 2 and 5 days of life in healthy term neonates and in hypocalcemic preterm infants using a new immunoradiometric assay which measures only biologically active intact parathyroid hormone and by a mid-molecule parathyroid hormone radioimmunoassay. During pregnancy intact and mid-molecule parathyroid hormone levels did not show any modification and were not different from parathyroid hormone levels of nonpregnant age-matched controls. Serum calcium and phosphorus levels did not vary during each trimester of pregnancy. In cord serum intact and mid-molecule parathyroid hormone values were low in both term and preterm infants. In term neonates intact and mid-molecule parathyroid hormone levels peaked on day 1; in preterm infants intact parathyroid hormone levels peaked on day 1 while mid-molecule parathyroid hormone values peaked on day 2. Intact parathyroid hormone levels showed a more marked increase in preterm (19-fold) than in term neonates (7.5-fold) on day 1. Our data do not confirm the previously reported "physiologic" hyperparathyroidism in pregnancy. Moreover we found a normal parathyroid gland responsiveness to decreasing serum calcium levels in the first days of life in term and preterm infants. Our results suggest that measurement of intact parathyroid hormone 1-84 by immunoradiometric assay in the first days of life is a more sensitive index of parathyroid gland secretory function than the measurement of middle or carboxyl-terminal parathyroid hormone fragments allowing the detection of the dynamic changes of parathyroid hormone which occur in hypocalcemic preterm infants.

Maternal Mineral and Bone Metabolism During Pregnancy, Lactation, and Post-Weaning Recovery

“…Serum phosphorus shows no change in most studies during human pregnancy (33,207,221,294,468,632,641,674,791) and was similarly unchanged during twin pregnancies (646). There is also no change in the renal tubular reabsorption of phosphorus (207,329,468).…”

“…Later innovations resulted in the ability to measure the ionized calcium with ion-specific electrodes. Both cross-sectional (210,227,324) and longitudinal studies (221,294,721,739,791,809,813) have shown that the ionized calcium does not change during human pregnancy, even though the mean serum calcium simultaneously drops ϳ10% into the hypocalcemic range (221).…”

“…In cross-sectional studies of North American and European women consuming a diet adequate in calcium, PTH is typically suppressed to the lower end of the normal range during all three trimesters (207,227,294,324,791,809,905). In longitudinal studies from North America, Europe, and Asia, the mean PTH level was suppressed to the lower end of the normal range or below it during the first trimester, and either remained suppressed or increased to mid-normal by the end of pregnancy (33, 85,207,221,304,632,739,813).…”

Bone Development and Mineral Homeostasis in the Fetus and Neonate: Roles of the Calciotropic and Phosphotropic Hormones

“…In human babies, PTH also circulates at low levels compared with the maternal circulation and normal adult values; very low (Ͻ0.5 pM) concentrations may be observed (20, 143, 168, 177, 255, 306, 501, 544, 571, 580, 598 -600, 671, 736). This suppression of PTH has been observed as early as 19 wk of gestation in preterm babies (501,571,580,599), although one study found unsuppressed PTH values in cord blood of infants aged 31-36 wk (152). The suppression of fetal PTH is considerable when it is realized that the maternal PTH value is also usually suppressed during pregnancy compared with normal adult values, at least in women from North America and Europe (328,339).…”

“…Subsequent studies used intact PTH assays but obtained measurements solely at birth and 24 h later, and demonstrated that PTH rose significantly (31,386,388,416,444,473,580). Those studies did not determine when the PTH value increased or whether the peak level was achieved at 24 h or later.…”